As technology development and demand for mobile devices increase, demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, a lithium secondary battery which exhibits high energy density and voltage, a long cycle lifespan, and a low self-discharge rate has been commercialized and widely used. Also, research is being actively conducted on a method of manufacturing an electrode having higher energy density per unit volume by improving electrode density, such as an electrode for a high-capacity lithium secondary battery.
Generally, since a high-density electrode is formed by molding electrode active material particles having a size of several to several tens of μm through a high-pressure press, it is easy to deform particles, reduce a space between particles, and degrade permeability of an electrolyte solution.
In order to solve these problems, a conductive material having excellent strength in addition to excellent electric conductivity has been used to manufacture an electrode. When a conductive material is used to manufacture an electrode, a conductive material is dispersed between compressed electrode active materials, and thus a micropore between active material particles is maintained so that an electrolyte solution may be easily permeated, and conductivity is also excellent so that resistance in an electrode may be reduced. Among such conductive materials, carbon nanotubes as a fibrous carbon-based conductive material capable of further decreasing electrode resistance by forming an electroconductive pathway in an electrode have been increasingly used in recent years.
Carbon nanotubes, which are a type of carbon microfiber, are a form of tubular carbon having a diameter of 1 μm or less, and are expected to be applied to and practically used in various fields due to high conductivity, tensile strength, heat resistance and the like, which result from a specific structure thereof. However, despite their usefulness, carbon nanotubes have limited applications due to low solubility and dispersibility. That is, carbon nanotubes are not stably dispersed in an aqueous solution due to strong van der Waals forces between each other, which causes an agglomeration phenomenon.
In order to solve these problems, there have been various attempts. Specifically, a method of dispersing carbon nanotubes in a dispersion medium through a mechanical dispersion process such as sonication or the like has been proposed. However, this method has a problem in which dispersibility is excellent during irradiation of ultrasonic waves, but carbon nanotubes start to be agglomerated when the irradiation of ultrasonic waves is terminated, and are agglomerated as a concentration of carbon nanotubes is higher. Also, a method of stably dispersing carbon nanotubes using various dispersants has been proposed, but this method also has a problem of handling due to an increase in viscosity when carbon microfiber is dispersed at high concentration in a dispersion medium.
Accordingly, a preparation of carbon nanotubes having excellent dispersibility as a conductive material for a secondary battery is necessary.